A word of caution. In two serious commercial enterprises where I have worked, osCommerce just didn't cut it. it's fine for a very simple shop business model, but if you have a more complicated business model and need to include a lot by way of custom business logic, it can take a lot to whip osCommerce into the shape you need.
osCommerce's abstraction is quite poor. The distinctions between presentation, business, service and database layers are not very clear and you can find yourself on a bit of an exploration trip to find the code you need to change.
